This invention relates to an automatic apparent density measuring device which is especially effective for such product as potato chips or cornflakes having variable apparent density or specific gravity. The inventive device is useful when applied to an automatic weighing machine for weighing product in both weight and volume for the purpose of controlling a preset target weight based upon a measured density.
U.S. Pat. No. 4,548,286 discloses a combination weighing machine which can weigh out a quantity of product having a weight equal or approximate to a preset target weight and also a volume falling within a preset allowable range. This machine includes a plurality of weighing hoppers for weighing product and corresponding pool hoppers for temporarily holding a selected amount of product before feeding it to the weighing hoppers. One of the pool hoppers is provided with a photoelectric detector for detecting the level or height and consequently the volume of product fed therein. The detected volume is combined in a separate calculating unit with the corresponding weight measured by the underlying weighing hopper to obtain the apparent density. The calculated apparent density is supplied to a target weight control unit for controlling a preset target weight based upon the density so that the volume of each delivery of product from the machine falls within a preset allowable range.
However, the method and device for measuring apparent density in the above U.S. patent has the following disadvantages. First, measuring volume by detecting the height of product which has fallen naturally or spontaneously into the pool hopper from a feeding trough may be substantially imprecise especially when the product is variable in shape and particle size as in the case of fragile product such as cornflakes or potato chips. The error will be small when the product is uniform in shape and size; however, in this case the density should be uniform so that target weight control would not be needed. Second, there is a substantial chance of delivering a batch outside the allowable range of volume, for example, when a sudden change in density takes place, since the target weight control is effected on the basis of the density of current delivery.